Are you after modify_header()?

library(dplyr)
library(gtsummary)
library(ggplot2)

mpg %>%
  select(manufacturer, drv) %>%
  tbl_summary(by = drv,
              digits=list(all_categorical() ~ c(function(x){round(x/5) * 5}, 0)),
              type = list(displ = "continuous2")) %>%
  modify_header(all_stat_cols() ~ "**{level}** \n\n **N = {round(n/5)*5}**")

Gives:

(output truncated)